Critical carbonate minerals in geochemical prospecting

ABSTRACT

In a geochemical prospecting method, samples are analyzed for a critical carbonate mineral which contains most of the hydrocarbons in the sample. The mineral is almost always dolomite, calcium-magnesium carbonate, but may be other carbonate minerals, such as iron carbonate, or even calcium carbonate. The critical carbonate should be separated from any other carbonates which are present in significant amounts. Gases are then released from the critical carbonate. Released gases are analyzed for hydrocarbon content. Preferably, gases are released by acid treatment. These gases are then analyzed for carbon dioxide content as a measure of the carbonate mineral. The ratio of hydrocarbons per unit of critical mineral is then plotted to form a geochemical prospecting map.

United States Patent [191 Thompson et al.

[ CRITICAL CARBONATE MINERALS IN GEOCHEMICAL PROSPECTING [73] Assignee:Amoco Production Company, Tulsa,

Okla.

[22] Filed: Dec. 10, 1971 [21] Appl. No.: 206,813

[52] U.S. Cl. 23/230 EP, 23/232 R [51] Int. Cl. G0ln 33/24 [58] Field ofSearch 23/230 EP, 232 R, 232 E;

[56] References Cited UNITED STATES PATENTS 7/1939 Bays 23/232 R l/l942HOl'VitZ 23/232 R OTHER PUBLICATIONS Peterson et al., QuantitativeDetermination of Calcite and Dolomite in Pure Carbonates and'Limestones,

1 ,115. 3,801,281 "[451 Apr. 2, 1974 Jour. of Soil Science, vol. 17, No.2, p. 317-327 (Sept. 1966). I

Primary Examiner.loseph Scovronek Attorney, Agent, or FirmNewellPottorf; Paul F. Hawley [57] ABSTRACT In a geochemical prospectingmethod, samples are analyzed for a critical carbonate mineral whichcontains most of the hydrocarbons in the sample. The mineral is almostalways dolomite, calcium-magnesium carbonate, but may be other carbonateminerals, such as iron carbonate, or even calcium carbonate. 'Thecritical carbonate should be separated from any other carbonates whichare present in significant amounts. Gases are then released from thecritical carbonate. Released gases are analyzed for hydrocarbon content.Preferably, gases are released by acid treatment.

7 These gases are then analyzed for carbon dioxide content as a measureof the carbonate mineral. The ratio of hydrocarbons per unit of criticalmineral is then plotted to fonn a geochemical prospecting map.

6 Claims, 2 Drawing Figures PAIENTEDAPR 21914 3.801.281

sum 1 or 2 EPB in TOTAL SAMPLE(ppb by wt) 0 2 MILES SALT DOMEPATENTEDAPR 2mm 3.801.281

snm 2 or 2 I EPB in DOLOMITE (ppb by wt) o I 2 MILES CRITICAL CARBONATEMINERALS IN GEOCI'IEMICAL PROSPECTING In geochemicalprospecting methods,soil samples are obtained and analyzed for hydrocarbon gas content. Theconcentration may then be expressed in terms of parts of hydrocarbon gasper unit weight of sample. It has long been recognized that thisexpression of concentration gives erratic results. It has also beenproposed that the erratic results are due to lithologic differencesbetween samples.

Various methods have been devised by other workers in an attempt toovercome the problem. These include methods based on correcting the rawdata according to the sorptive power of soil samples, as suggested inU.S. Pat. No. 2,165,440 Bays, correcting in terms of total acid solubleminerals as proposed in Canadian Pat. No. 723,550 Debnam, correcting forsurface area as described in U.S. Pat. No. 3,120,428 McDermott, andaccording to gross lithologic differences as disclosed in French Pat.No. 1,448,079.

Even results corrected by the previously proposed methods usually giveresults which are erratic to an undesirable degree. Therefore, an objectof this invention is to provide a correction technique which will giveless erratic results. Still other objects will be apparent from thefollowing description and claims. I

SUMMARY OF THE INVENTION We have found that a very high percentage ofhydrocarbons is present in a single type of carbonate mineral. This isalmost always dolomite, calcium magnesium carbonate. Therefore, in orderto avoid erratic results, samples must be analyzed for this mineral andthe hydrocarbon concentration calculated with respect to this specificmineral rather than with respect'to total sample weight, or even totalcarbonates.

In the drawing,

FIG. 1 shows a geochemical prospecting map of an area near the Gulf ofMexico coast. This map was plotted using values calculated on the basisof parts of the sum of ethane, propane and butane per billion parts oftotal sample.

FIG. 2 shows another geochemical prospecting map of the same area. Thismap was plotted using values calculated on the basis of parts of thesame hydrocarbon gases per billion parts of dolomite.

FIG.1 shows only a general change from very low' values in the northpart to higher values covering a broad trend in the south part. Thehighest values (30 PPB HYDROCARBONS, Screened Sample Basis 20 ever ofthis anomaly.

When our method was applied to this prospect, it was, found that thebulk of the hydrocarbon gases in the samples was contained in dolomite.The map shown in FIG, 2 was then prepared showing the sum of concen- 5trations of the three light hydrocarbons, ethane, propane and butane interms of the dolomite in each sample. In this case, the values over thedome and its flanks (3,000 to 4,000 ppb) are in the range ofconcentrations of hydrocarbons in dolomite over known petroleumreservoirs in this area. This dolomite-corrected map Since the surveywas run, wells on FIG. 2 have been drilled resulting in the discovery ofa major oil field.

FIG. 2 also shows an anomaly in the northwest corner of the areacovered. FIG. 1 gives no indication what- An anomaly is also shown inthe southwest corner of FIG. 2. This appears to be associated with asalt dome just outside the area covered. This dome, also undrilled atthe time of the survey, has since developed into a ,r s fis il l The mapshown in FIG. 2 is the result of data obtained using ethylene diaminetetraacetic acid (EDTA) to separate calcite from dolomite. This step wasnecessary in the analysis for dolomite content. As explained later, italso avoids an error by removing any hydrocarbons held by thecalcite.The method was much like that described in Method of Solution ofCalcareous Materials Using the Complexing Agent, EDTA, by

Everett D. Glover, Journql of Sedimentary Petrology,

Volume 31 (l'961),'pages 622-626. In the article,

EDTA is referred to as ethylene diamine dinitrilo tetracetic acid but isactually the same EDTA currently more commonly called simply ethylenediamine tetrabonates and'Lirnestonesf by G. W. Petersen et al.,

Journal of Soil Science, Volume l7 (1966),pages 317-327. In this method,the calcite and dolomite were separated, the'hydrocarbon content of eachfractionv being measured. "Using this technique, the results TABLE I PPBHydrocarbons, Fraction Basis Dolomite Calcite Dolomite Calcite MethaneEPB Sample 7: 7: Methane EPB Methane EPB Dolomite Calcite DolomiteCalcite values to 1,000 or more parts per billion) usually measured overoil or gas reservoirs in the area. This and using citric The samplesreported in Table I were taken in a small area off the shore ofLouisiana. This group was selected the fact that the higher values coversuch a broad area 65 for presentation since the analyses illustrateclearly the around the salt dome prospect made it uncertain whether atrue anomaly was present or only a background change.

wide difference in concentration of hydrocarbons held by the twominerals. EPB stands for ethane, propane and butane. In the table, themethane and EPB content of the dolomite and of the calcite are firstpresented on the basis of the portion of the total sample passingthrough a No. 230 U.S. standard sieve, the concentrations beingcorrected toa dry basis. The concentrations of methane in the dolomiteand calcite are then combut the number of samples was much greater andsome of the concentrations of dolomite and calcite were much lower.Sample 16 is of particular interest in showing that when the totalcarbonate content of samples is low, so is hydrocarbon content. Thereare exceptions to thisrule, but they seern to be rare. Lower dolomite '4calcite, the same general rule obviously applies asin Table I; that is,hydrocarbon concentrations in dolomite are much greater than in calcite.

in the results of Tables I and ll, dolomite is obviously the criticalcarbonate mineral. This is almost always the pared, followedbyacomparison between the contents case Th h b a very f s, h wever, Ofin dolomite and calcite in thfi fine fraction Of where other carbonateshavg appeared to be the cfifieach sample. To obtain the latter values,the concentracarbonate I n case, for example, a sample was hohs aScreened 53mph? basis were p y divided by found to contain considerablehydrocarbons but X-ray the dolom1te and calc1te concentrat ons in thesample.- diff ti i di d h Sample contained very li l It be pp h from thedatafh Table I that the dolomite. The X-ray diffraction also showed thesample hydrocarbon content of the dolomlte a factor of at containedconsiderable siderite (iron carbonate). Partileast ten greater than thehydrocarbon content of the C188 in the micron range were isomted by acahmeslhce the calcte fohcemrahoh 'freqhemly combination of screeningand elutriation. A heavy liqgreafly e xce eded thedoiomlte cohcehtrahohthe uid-density separation technique was used to separate Sample, beobvious that bashg ahalysls oh dolomite and siderite fractions. Thedolomite fraction total carbonates rather than on dolomite alone wouldcontained only about 7,700 ppb f EPB, while the f h @8 error In thevalueserite contained about 12,100 ppb of EPB.

R poss'ble to analyze total Sample for total in another case, sampleswere taken on shore in the methane and EP content h correct thhse Texas-Gulf Coast area. X-ray diffraction showed at h dolomleCQhCemratOh';However h least some samples contained no dolomite orsiderite l"? lhcludeshydmcarbohs whlch ajctuahy P (less than 0.3 percentin the method used), but conm other mlheralsishch as calclte- Thlsi ofcom, tained considerable calcite. Results of analysis are 1ntroduces anerror. Smcethe fracnonof total hydrogiven in m m carbons present 1nother mmerals is usually small com- 7 pared to those in dolomite, theerror generally is not great. lt is preferred, however, to eliminatethis error by A m separating the dolomite from other carbonate mineralsand-then releasing the hydrocarbons present only inthe v dolomiteMineral Concentrations, EPB- Sample No. Calcite Dolomite Sidcrite Cone.PPB

Table ll 15 presented to overcome the somewhat m1sleading impressionfrom Table I that the hydrocarbon i; :2 g;

i one CBC content of dolom1te always greatly exceeds that of cal- 26None None 130 c1te.; V 27 30 2 3 None 86 The samples in Table ll arefrom a different area in 35 28 N099 None 29 None 3-4 None 0.5 the samegeneral offshore region as those in Table l,

249 511 PPB HYDROCARBONS, Screened Sample Basis PPB Hydrocarbons,Fraction Basis Dolomite Calcite Dolomite Calcite Methane EPB Sample 7:7c Methane EPB, Methane EPB Dolomite Calcite Dolomite Calcite 6 2.9 0.951,893 382 I 57 18 65,400 6,000 13.150 1,895 6 2.7 1.3 1,827 .365 147 3267,600 11,300 13,510 2,460 ,7 2.6 0.79 1.650 348 79 21 63,500 10,00013,400 2,660 7" 2.7 0.77 1.454 319 66' 19 53,900 8,570 11,800 2,470 80.11 0.023 I 23 3 10 20,900 174,000 2,720 43,500 9 2.1 1.3 1,198 229 5911 57,000 4,540 7 10,900 847 10 2.5 0.073 1,032 174 3 0 41,300 4,1006,970 0 11 2.4 0.66 537 31 8 22,400 4,700 4,330 1.210 12 2.9 1.4' 1.439274 102 23 49,600 7,290 9,450 1,640 13 1.9 1.6 653 129 49 12 34,4003,060 6,800 750 14 0.19 0.78 72 13 5 37,900 i 641 9,480 123 15 0.44 1.4179 38 35 9 40.700 2.500 8,640 642 16 0.000 0.001 6 1 2 0 17 0.18 05621s 45 39 121,000 25,000 25,000 6,970 18 2.3. 1.4 1,297 235 197 4456,500 14,100 10,200 3,140 19 0.28 1.1 59 2s 3s 88 21.100 3,450 10,0008,000

The EPB concentrations in Table III are based on the fraction of thetotal sample between 63 and 4 microns in particle size.

A general correlation between calcite and EPB concentrations is obvious.There is no apparent correlation between EPB and dolomite, particularlyin samples 29 and 30. From the data in Table III, it would seem thatcalcite is the critical carbonate mineral in this area.

As previously noted, dolomite is almost always the critical carbonatemineral but there can be exceptions.

For this reason, it is usually advisable to make a preliminaryanalysisof at. least some of the sample by some means, such as X-raydiffraction, todetermine how much of the various carbonate minerals arepresent. In a very large majority of cases, the principal carbonateminerals are dolomite and calcite. In this case, the followinganalytical procedure is preferred.

First, the soil samples are wet-screened using a No. 230 U.S. standardsieve. This removes particles larger than about 63 microns, so thedifferential solution technique using citric acid will not be toogreatly upset by wide differences in particle size. The slurry of smallparticles is then filtered and a portion of the resulting fiber cake isanalyzed for water content.

Next, another portion of the filter cake is placed in a reaction flaskwith distilled water, the flask is attached to vacuum equipment andimmersed in a water bath at-about 70 to 80 F. The apparatus is thenevacuated after which it is isolated from the vacuum pump.

Citrate buffer solution is then added. This is citric acid solutionadjusted to a pH of about 5.85 with ammonium hydroxide. The amount ofcitrate solution should be at least two or three times that necessary todissolve the calcite which is present. The reaction is allowed toproceed for about 20 minutes, after which the water bath is replaced byan ice bath. The buffered citrate solution at 70 to 80 F dissolvesalmost all of the calcite, releasing any calcite contained hydrocarbonsand forming carbon dioxide. .T he solution dissolves only a few percentof the dolomite. The ice bath stops,

or drastically decreases, any continued reaction with dolomite. Afterthe flask has cooled for about five minutes, the gases are withdrawnfrom the apparatus and analyzed for methane, EPB, and carbon dioxide.

Theevacuated apparatus is again isolated from the of CarbonateSediments; Comparison of X-ray Diffraction and ElectronProbe-Microanalyzer Methods," by J. N. Weber, Journal ofSedimentaryPetrology, Volume 38 (1968), No. 1, pages 232-234.

Analysis for hydrocarbons can be by any of the many techniquespreviously disclosed, use of a hydrogen flame ionization detector behinda chromatographic column being preferred.

Preferably, the critical carbonate should be separated at least fromother carbonates before release of hydrocarbons. This separation can beby differential solution, heavy liquid-density separation, or. the like.This permits calculation of the concentration of hydrocarbons held onlyby the critical carbonate. As previously noted, however, a largemajority of the'hydrocarhens in the soil sample is usually held by thecritical carbonate. Therefore, inclusion of hydrocarbons held by otherminerals usually causes only minor errors. For this reason, plotting oftotal hydrocarbons in a sample per unit of critical carbonate ordinarilyproduces a useful geochemical prospecting map. 7

Still other variations and alternates will be apparent to those skilledin the art. Therefore, we do not wish to be limited to the examplesdescribed above, but only by the following claims.

We claim:

1. In a process for geochemical prospecting in which soil samples aretaken over an area and are analyzed for hydrocarbon content,theimprovement comprising analyzing each sample for the content ofthecritic al carbonate mineral which contains a majority 'of thehydrocarbons present in said sample and plotting a map showing theconcentration of at least one hydrocarbon per unit of critical'carbonatemineral in each sample.

pump and the ice bath is replaced by the water bath.

Next, hydrochloric acid is added to the reaction flask and reaction isallowed to proceed for about an hour. This dissolves the dolomite,releasing dolomite contained hydrocarbons and forming carbon dioxide.Thewater bath is replaced by an ice bath and after about 5 minutes thegases are withdrawn and analyzed for hydrocarbonand carbon dioxide.

The amount of dolomite is calculated from the released carbon dioxide.This permits calculating the concentration of hydrocarbons per unit ofdolomite in each sample. These concentrations are then plotted on a mapto form the desired geochemical prospecting map.

There are many alternates and variations to this preferred technique.Basically, our process simply requires analyzing a sample for thecritical carbonate mineral (almost always dolomite) and forhydrocarbons, so the concentration of hydrocarbons per unit of dolomitein the sample can be calculated and plotted.

Analysis for dolomite can be by chemical means or by other means, suchas infrared spectroscopy. See, for example, The Application of InfraredAbsorption Spectroscopy toCarbonate Mineralogy," by R. Chester et al.,Sedimenrology, Volume 9 (1967), No. 1, pages 5-21. Analysis may also bemade by X-ray diffraction. See, for example, Quantitative MineralogicAnalysis unit of critical carbonate mineral in the fine fraction thenbeing plotted to form the geochemical prospecting map.

4. The method of claim 1 in which said critical carbonate mineral isseparated from at least other carbonate minerals before hydrocarbons arereleased from said critical carbonate mineral, the fraction containingsaid critical carbonate mineral is analyzed for hydrocarbon content andcritical mineral content, and the map is plotted showing theconcentration of at least one hydrocarbon per unit of critical carbonatemineral in said fraction.

5. The method of claim 4 in which said critical carbonate mineral isdolomite.

6. The method of claim 5 in which said sample is screened to rejectlarge particles, the fine fraction is treated first with controlled pHcitrate solution to remove calcite and is then treated with hydrochloricacid to release hydrocarbons held by the dolomite and carbon dioxide insaid dolomite, the amount of carbon diodixe then being measured todetermine the amount of dolomite in the fine fraction of the sample.

2. The method of claim 1 in which said critical carbonate mineral isdolomite.
 3. The method of claim 1 in which said sample is firstscreened to eliminate large particles, the concentration of hydrocarbonsand critical carbonate mineral in the fine fraction passing through thescreen then being determined, the amount of at least one hydrocarbon perunit of critical carbonate mineral in the fine fraction then beingplOtted to form the geochemical prospecting map.
 4. The method of claim1 in which said critical carbonate mineral is separated from at leastother carbonate minerals before hydrocarbons are released from saidcritical carbonate mineral, the fraction containing said criticalcarbonate mineral is analyzed for hydrocarbon content and criticalmineral content, and the map is plotted showing the concentration of atleast one hydrocarbon per unit of critical carbonate mineral in saidfraction.
 5. The method of claim 4 in which said critical carbonatemineral is dolomite.
 6. The method of claim 5 in which said sample isscreened to reject large particles, the fine fraction is treated firstwith controlled pH citrate solution to remove calcite and is thentreated with hydrochloric acid to release hydrocarbons held by thedolomite and carbon dioxide in said dolomite, the amount of carbondiodixe then being measured to determine the amount of dolomite in thefine fraction of the sample.